Planetary ring
A planetary ring is a ring of cosmic dust and other small particles orbiting around a planet in a flat disc-shaped region. The most spectacular planetary rings known are those around Saturn, but the other three gas giants of the solar system (Jupiter, Uranus and Neptune) possess ring systems of their own. Recent reportshttp://www.nasa.gov/mission_pages/cassini/media/rhea20080306.html NASA - Saturn's Moon Rhea Also May Have Rings have suggested that the Saturnian moon Rhea may have its own tenuous ring system, which would make it the only moon known to possess a ring system. Overview There are three ways that planetary rings have been proposed to have formed: from material of the protoplanetary disk that was within the Roche limit of the planet and thus could not coalesce to form moons; from the debris of a moon that was disrupted by a large impact; or from the debris of a moon that was disrupted by tidal stresses when it passed within the planet's Roche limit. Most rings are thought to be unstable and to dissipate over the course of tens or hundreds of millions of years, but it appears that Saturn's rings might be quite old, dating to the early days of the Solar system. The composition of ring particles varies; they may be silicate or icy dust. Larger rocks and boulders may also be present, and in 2007 tidal effects from eight 'moonlets' only a few hundred meters across were detected within Saturn's rings. Sometimes rings will have "shepherd" moons, small moons that orbit near the outer edges of rings or within gaps in the rings. The gravity of shepherd moons serves to maintain a sharply defined edge to the ring; material that drifts closer to the shepherd moon's orbit is either deflected back into the body of the ring, ejected from the system, or accreted onto the moon itself. Several of Jupiter's small innermost moons, namely Metis and Adrastea, are within Jupiter's ring system and are also within Jupiter's Roche limit . It is possible that these rings are composed of material that is being pulled off of these two bodies by Jupiter's tidal forces, possibly facilitated by impacts of ring material on their surfaces. A moon inside the Roche limit is held together only by its mechanical strength rather than by its gravity, and so loose material on its surface would simply "fall off" to join the rings . Neptune's rings are very unusual in that they first appeared to be composed of incomplete arcs in Earth-based observations, but Voyager 2's images showed them to be complete rings with bright clumps . It is thought that the gravitational influence of the shepherd moon Galatea and possibly other as-yet undiscovered shepherd moons are responsible for this clumpiness. Pluto is not known to have any ring systems. However, some astronomers think that the New Horizons probe might find a ring system when it visits in 2015. It is also predicted that Phobos, a moon of Mars, will break up and form into a planetary ring in about 50 million years due to its low orbit. Gürtler, J. & Dorschner, J: "Das Sonnensystem", Barth (1993), ISBN 3-335-00281-4 After the impact of Theia and before the coalescence of the Moon, it is generally assumed that the Earth had a ring system. Visual comparison '' image of Jupiter's main ring.]] '' image of Uranus' rings.]] '' images of Neptune's rings.]] See also * Rings of Jupiter * Rings of Saturn * Rings of Uranus * Rings of Neptune * Rings of Rhea External links * USGS/IAU Ring and Ring Gap Nomenclature Notes * Category:Celestial mechanics Category:Granular materials